This invention relates to x-ray systems employing a solid state detector and more particularly relates to such systems in which data of interest occupies less than all elements of the detector.
Experience has shown that a modular design approach to x-ray detector control helps reduce the time to market. By using this approach, a common set of modular electronics may support three different detectors, such as a 41 cm square Radiographic detector, a 20 cm square Cardiographic detector, both consisting of square 0.2 mm pixels; and a Mammographic detector that is 23 cmxc3x9719.2 cm, consisting of square 0.1 mm pixels. The support electronics, consisting of drive modules to control the detector""s field effect transistor (FET) switches and sense modules to read and convert the signal, may be 256 channel subsystems, designed to support detectors with 0.2 mm pixel pitch. Both the Cardiographic and Radiographic detectors may require an exact integer number of modules. The Mammographic detector, however, is different in a number of respects. It is rectangular and the pixel pitch is 0.1 mm. The smaller pitch is accommodated by sensing alternate channels from opposite sides, reducing the effective pitch of the sense electronics from 0.2 mm to 0.1 mm. The drive modules are similarly attached, although from two sets of (alternating) contacts that are on the same side in order to allow patient access to one edge of the detector. At first glance it would appear that both a non-integer number of drive and sense modules would be required. However, an exact multiple of 256 sense channels is required across the 19.2 cm dimension. The drive modules however would support 2560 channels, while only 2304 are required. An even number of modules may be required due to the manner in which the pitch mismatch between the detector and the module is managed, even though the requirements suggest that the problem could be solved with 9 (rather than 10) drive modules. This means that 128 drive channels are not committed to the detector on each end.
Given that the drive module consists of a custom Application Specific Integrated Circuit (ASIC), designed to function like a serial shift register with high voltage outputs, at least the first 128 uncommitted driver channels will require consideration during read out of the detector. The last 128 channels can be ignored due to the operation of module reset, which allows asynchronous reset of the shift register. One current FFDM x-ray detector is operated as if it consisted of 2432 scan lines. That is, the first 128 uncommitted drive channels are operated just like the following 2304 drive channels that are attached to detector scan lines, with the exception that the corresponding xe2x80x9cimagexe2x80x9d data is not transferred out of the detector. This is because this data does not contain any X-Ray exposure information. However, operating the drive electronics in this manner imposes more than 5% of useless overhead (time).
Similarly, when smaller fields of view than the actual size of the detector are required, the scan lines not in the field of view will require scrubbing (that is the detector is read to restore the charge for each pixel, but the data is discarded). If the FETs are not scrubbed or read for long periods of time, the threshold voltage shifts (in an irreversible fashion) and the FETs no longer provide the necessary isolation when they are off, or conversely, may not provide low enough impedance when on to allow the pixel charge to be quickly and thoroughly restored, resulting in erroneous signal conversion in either case. If the scan lines outside the field of view are scrubbed in a fashion similar to the uncommitted drive channels of the FFDM detector, no advantage in read out time will be enjoyed by defining a smaller field of view on a larger detector. In order to support higher acquisition rates, a smaller detector would need to be substituted, which is a disadvantage in comparison to present Image Intensifier based systems which can easily switch between different field sizes. Conversely, a very complex readout algorithm shared between the detector and the X-Ray system could be defined which would be prone to error, corrupting the image acquisition in the process.
U.S. Pat. No. 4,996,413 discloses a split image x-ray detector that is read from the middle to the outside. Although this reading technique was an improvement in the art, subsequent research has shown that it is desirable for at least some applications to read the detector from the outside toward the middle. The present invention provides a technique for reading detectors that is an improvement on the teaching of U.S. Pat. No. 4,996,413.
The present invention addresses the foregoing problems and provides a solution.
The preferred embodiment is useful in an x-ray system for reading data from a detector array comprising detector elements arranged in rows and columns. The rows typically comprise a first plurality of rows including unneeded data and a second plurality of rows including data of interest. In such an environment, apparatus for coordinating the activation of the detector elements in relation to the exposure of a patient to x-rays to improve the efficiency with which the data of interest is read preferably comprises an x-ray tube generating x-rays. An exposure control is arranged to activate the x-ray tube to expose the detector to x-rays during a predetermined first time period. A detector controller is arranged to activate the first plurality of rows at least partially before or during the first time period and to activate the second plurality of rows after the first time period. An image processor is arranged to read the data in the second plurality of rows after the first time period.
Another embodiment includes a method analogous to the above-described apparatus.
By using the foregoing techniques, the overhead associated with scrubbing or partial read out of solid-state x-ray detectors can be substantially reduced.